how to study effectively with official questions

Depends on what official material you are talking about - OG13/Review Guides/Prep tests/Question Pack

If you are talking about OG13 or Review guides, and if you have done the questions a couple of times, you will not get any real benefit out of them. Not because there isn't real benefit in them but because
- you will not get the feel that a new question gives you. Instead you will feel the comfort you feel from familiarity.
- most questions of these sources are up to 650 level so if you want to see harder questions, these sources are not very useful.
- if you have already done them twice, you may not put your heart into analyzing them properly - the thing that actually gives all the benefit
You can review the questions you found hard last time - if you have marked them - to ensure that now, you do get the logic.

On the other hand, the prep tests and the question packs do have harder questions and there might still be plenty of questions in them with which you may not be comfortable. You should re-review them, try to solve them on your own using multiple methods, check out the solution, log your takeaways etc.

For RC, again, new passages will bring in most value. Once you understand a piece of writing, it seems very reasonable and logical to you. But first reads are often confusing and need more time. For some serious practice, I would suggest you to look for new material. You can come back to the hard official questions once you see improvement. They will still not give you a realistic picture since you have already done them twice so don't get complacent.

Check out our free question bank for lots of new questions: https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#-question-bank/